The Mind-Flayer is absent from the Upside Down's skyline in the fourth season of Stranger Things, so it's unclear if he's actually a five-star general. The show's fourth season features a powerful human being who has been living in the Upside Down, rather than a mindless creature like the Demogorgon or a monstrous intelligence like the Mind-Flayer. Steve, Robin, Eddie, and Nancy didn't see any sign of the Mind-Flayer when they were in the Upside Down. One possible explanation is that Vecna moves the two dimensions closer together so that he can open a new Gate between the two. It's clear that a mental contact across the dimensions is key to opening Gates to the Upside Down, so this makes sense.
Vecna's curse takes advantage of the trauma that Hawkins has suffered over the years, not all of which is connected to the Upside Down. It's reasonable to assume that the final act consumes the most energy, with Vecna killing his victim and tearing open the Gate. Number One was blasted into the Upside Down by the real inciting incident of their battle, with Eleven triumphing but tearing open a window through the dimensions. In the past, the Mind-Flayer has taken advantage of any Gates to launch direct invasions on Hawkins; now there are three Gates, but there is no hint of an invasion.
Vecna appears to draw upon the life energy of the entire Upside Down hive while projecting himself across the dimensions; his actions cause powerful flares of energy that are visible even from Hawkins. The Gate to the Upside Down has been torn open by Vecna's killings, potentially allowing creatures from that dangerous realm to make their way into their town. The Gate is only created when Vecna's victim is fully immersed in his vision, which explains why there aren't many Gates scattered across Hawkins as a result of Vecna's gentlest psychic touch.
Vecna revels in the pain and suffering he is causing, but that seems to be a side-effect of the process, rather than its core component. The war between the worlds is set up in the first volume of the fourth season of the show. The basic principle seems to apply despite the fact that the first Gate was created when Eleven touched the Demogorgon.
Vecna can only kill his victims when they are completely immersed in the vision of his mind. Vecna's strange grandfather clock is sometimes used to project visions into the minds of his victims. A vision that began with her guilt and shame over the death of her step-brother Billy in season 3 was interrupted by the kids saving Max by playing her favorite song. Vecna's curse appears to be the key to the fourth season of Stranger Things. "Vecna" is a being that makes its presence felt in the real world through horrible visions that they liken to a curse.
The kids think Vecna is working for the Mind-Flayer. It is possible that Vecna has taken over the hive mind and is working on his own plan.